The advantages of using a network comprised of switching elements which route packets of data through the network on the basis of an address contained within the packet are known. Such networks are commonly called binary routing networks. A specific example is the butterfly or banyan network. In this type of network, there exists only one unique path between any two network terminations. Networks of this type are used to interconnect a relatively small number of customer terminals which are normally digital computers. Fault detection is performed by each computer upon receipt of a packet by checking the packet for errors to determine if a transmission fault had occurred within the network. The fault detection scheme gives acceptable performance in the prior art system because of the small number of computers interconnected by networks of this architecture. Such a testing scheme does not allow one computer to completely test the network because in the prior art system no one computer has access to all of the routes within the network.
In other prior art packet switching systems which were made up of computers interconnected by links with a link being terminated at each end by interface facilities, it was known to do loop-around testing from one computer through a second computer's interface facility and back to the first computer. However, the second computer was required to precondition the interface facility before this testing could take place. This procedure greatly increased the overhead associated with testing function, since the two computers were required to interact to perform one test.
A large packet switching system having hundreds of thousands of terminals interconnected by a number of networks of the banyan type would need a more sophisticated fault detection and isolation technique than those techniques known in the prior art. It would be desirable to have a central processor controlling the testing of each network. An example of a large packet switching system would be a system designed to replace the present telephone network as it exists in the United States. Further, it would be unrealistic to rely on techniques which perform fault detection and isolation at the customer terminals if, for example, those terminals were located in Calif. and N.Y. In addition, each network of such a system would represent a local telephone office or a toll switching office and would be an extremely large network which would be controlled by a central processor. In such a system, the central processor which controls a particular network should be able to do fault detection and isolation within that office's network and to test the transmission links which interconnect the local network with a distant network.
In view of the foregoing, there exists a need within a packet switching system comprised of banyan switching elements for techniques which allow a central processor to perform fault detection and isolation on all packet routes within each local network and with respect to the transmission links which interconnect the networks. In addition, the central processor must be able to perform this testing without having to precondition the various interface facilities.